diff --git a/src/sentry_json.c b/src/sentry_json.c
index 316868256..cf02c7f1b 100644
--- a/src/sentry_json.c
+++ b/src/sentry_json.c
@@ -101,13 +101,49 @@ write_str(sentry_jsonwriter_t *jw, const char *str)
     sentry__stringbuilder_append(jw->sb, str);
 }
 
+// The Lookup table and algorithm below are adapted from:
+// https://github.com/serde-rs/json/blob/977975ee650829a1f3c232cd5f641a7011bdce1d/src/ser.rs#L2079-L2145
+
+// Lookup table of escape sequences. `0` means no need to escape, and `1` means
+// that escaping is needed.
+static unsigned char needs_escaping[256] = {
+    // 1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 1
+    0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 2
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 3
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 4
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, // 5
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 6
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 7
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 8
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 9
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // A
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // B
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // C
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // D
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // E
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // F
+};
+
 static void
 write_json_str(sentry_jsonwriter_t *jw, const char *str)
 {
     // using unsigned here because utf-8 is > 127 :-)
     const unsigned char *ptr = (const unsigned char *)str;
     write_char(jw, '"');
+
+    const unsigned char *start = ptr;
     for (; *ptr; ptr++) {
+        if (!needs_escaping[*ptr]) {
+            continue;
+        }
+
+        size_t len = ptr - start;
+        if (len) {
+            sentry__stringbuilder_append_buf(jw->sb, (const char *)start, len);
+        }
+
         switch (*ptr) {
         case '\\':
             write_str(jw, "\\\\");
@@ -142,7 +178,15 @@ write_json_str(sentry_jsonwriter_t *jw, const char *str)
                 write_char(jw, *ptr);
             }
         }
+
+        start = ptr + 1;
     }
+
+    size_t len = ptr - start;
+    if (len) {
+        sentry__stringbuilder_append_buf(jw->sb, (const char *)start, len);
+    }
+
     write_char(jw, '"');
 }
 
diff --git a/src/sentry_string.c b/src/sentry_string.c
index a1581c752..f745f6ad7 100644
--- a/src/sentry_string.c
+++ b/src/sentry_string.c
@@ -39,49 +39,6 @@ sentry__stringbuilder_reserve(sentry_stringbuilder_t *sb, size_t len)
     return &sb->buf[sb->len];
 }
 
-static int
-append(sentry_stringbuilder_t *sb, const char *s, size_t len)
-{
-    char *buf = sentry__stringbuilder_reserve(sb, len + 1);
-    if (!buf) {
-        return 1;
-    }
-    memcpy(buf, s, len);
-    sb->len += len;
-
-    // make sure we're always zero terminated
-    sb->buf[sb->len] = '\0';
-
-    return 0;
-}
-
-int
-sentry__stringbuilder_append(sentry_stringbuilder_t *sb, const char *s)
-{
-    return append(sb, s, strlen(s));
-}
-
-int
-sentry__stringbuilder_append_buf(
-    sentry_stringbuilder_t *sb, const char *s, size_t len)
-{
-    return append(sb, s, len);
-}
-
-int
-sentry__stringbuilder_append_char(sentry_stringbuilder_t *sb, char c)
-{
-    return append(sb, &c, 1);
-}
-
-int
-sentry__stringbuilder_append_char32(sentry_stringbuilder_t *sb, uint32_t c)
-{
-    char buf[4];
-    size_t len = sentry__unichar_to_utf8(c, buf);
-    return sentry__stringbuilder_append_buf(sb, buf, len);
-}
-
 char *
 sentry_stringbuilder_take_string(sentry_stringbuilder_t *sb)
 {
@@ -121,24 +78,6 @@ sentry__stringbuilder_set_len(sentry_stringbuilder_t *sb, size_t len)
     sb->len = len;
 }
 
-char *
-sentry__string_clone(const char *str)
-{
-    return str ? sentry__string_clonen(str, strlen(str)) : NULL;
-}
-
-char *
-sentry__string_clonen(const char *str, size_t n)
-{
-    size_t len = n + 1;
-    char *rv = sentry_malloc(len);
-    if (rv) {
-        memcpy(rv, str, n);
-        rv[n] = 0;
-    }
-    return rv;
-}
-
 #ifdef SENTRY_PLATFORM_WINDOWS
 char *
 sentry__string_from_wstr(const wchar_t *s)
diff --git a/src/sentry_string.h b/src/sentry_string.h
index feeff3678..255d55d17 100644
--- a/src/sentry_string.h
+++ b/src/sentry_string.h
@@ -22,25 +22,55 @@ typedef struct sentry_stringbuilder_s {
 void sentry__stringbuilder_init(sentry_stringbuilder_t *sb);
 
 /**
- * Appends a zero terminated string to the builder.
+ * Resizes the stringbuilder buffer to make sure there is at least `len` bytes
+ * available at the end, and returns a pointer *to the reservation*.
  */
-int sentry__stringbuilder_append(sentry_stringbuilder_t *sb, const char *s);
+char *sentry__stringbuilder_reserve(sentry_stringbuilder_t *sb, size_t len);
 
 /**
  * Appends a sized buffer.
  */
-int sentry__stringbuilder_append_buf(
-    sentry_stringbuilder_t *sb, const char *s, size_t len);
+static inline int
+sentry__stringbuilder_append_buf(
+    sentry_stringbuilder_t *sb, const char *s, size_t len)
+{
+    size_t needed = sb->len + len + 1;
+    char *buf = sb->buf;
+    if (!sb->buf || needed > sb->allocated) {
+        buf = sentry__stringbuilder_reserve(sb, len + 1);
+        if (!buf) {
+            return 1;
+        }
+    } else {
+        buf = buf + sb->len;
+    }
+
+    memcpy(buf, s, len);
+    sb->len += len;
+
+    // make sure we're always zero terminated
+    sb->buf[sb->len] = '\0';
+
+    return 0;
+}
 
 /**
- * Appends a single character.
+ * Appends a zero terminated string to the builder.
  */
-int sentry__stringbuilder_append_char(sentry_stringbuilder_t *sb, char c);
+static inline int
+sentry__stringbuilder_append(sentry_stringbuilder_t *sb, const char *s)
+{
+    return sentry__stringbuilder_append_buf(sb, s, strlen(s));
+}
 
 /**
- * Appends a utf-32 character.
+ * Appends a single character.
  */
-int sentry__stringbuilder_append_char32(sentry_stringbuilder_t *sb, uint32_t c);
+static inline int
+sentry__stringbuilder_append_char(sentry_stringbuilder_t *sb, char c)
+{
+    return sentry__stringbuilder_append_buf(sb, &c, 1);
+}
 
 /**
  * Appends an int64 value.
@@ -73,12 +103,6 @@ void sentry__stringbuilder_cleanup(sentry_stringbuilder_t *sb);
  */
 size_t sentry__stringbuilder_len(const sentry_stringbuilder_t *sb);
 
-/**
- * Resizes the stringbuilder buffer to make sure there is at least `len` bytes
- * available at the end, and returns a pointer *to the reservation*.
- */
-char *sentry__stringbuilder_reserve(sentry_stringbuilder_t *sb, size_t len);
-
 /**
  * Sets the number of used bytes in the string builder, to be used together with
  * `sentry__stringbuilder_reserve` to avoid copying from an intermediate buffer.
@@ -86,14 +110,28 @@ char *sentry__stringbuilder_reserve(sentry_stringbuilder_t *sb, size_t len);
 void sentry__stringbuilder_set_len(sentry_stringbuilder_t *sb, size_t len);
 
 /**
- * Duplicates a zero terminated string.
+ * Duplicates a zero terminated string with a length limit.
  */
-char *sentry__string_clone(const char *str);
+static inline char *
+sentry__string_clonen(const char *str, size_t n)
+{
+    size_t len = n + 1;
+    char *rv = (char *)sentry_malloc(len);
+    if (rv) {
+        memcpy(rv, str, n);
+        rv[n] = 0;
+    }
+    return rv;
+}
 
 /**
- * Duplicates a zero terminated string with a length limit.
+ * Duplicates a zero terminated string.
  */
-char *sentry__string_clonen(const char *str, size_t n);
+static inline char *
+sentry__string_clone(const char *str)
+{
+    return str ? sentry__string_clonen(str, strlen(str)) : NULL;
+}
 
 /**
  * Converts a string to lowercase.
diff --git a/src/sentry_transport.c b/src/sentry_transport.c
index 51f8e9b39..4ff909e03 100644
--- a/src/sentry_transport.c
+++ b/src/sentry_transport.c
@@ -3,6 +3,7 @@
 #include "sentry_envelope.h"
 #include "sentry_options.h"
 #include "sentry_ratelimiter.h"
+#include "sentry_string.h"
 
 #define ENVELOPE_MIME "application/x-sentry-envelope"
 // The headers we use are: `x-sentry-auth`, `content-type`, `content-length`