diff --git a/changelog/std-algorithm-iteration-joiner.dd b/changelog/std-algorithm-iteration-joiner.dd
new file mode 100644
index 000000000..0ae4b2796
--- /dev/null
+++ b/changelog/std-algorithm-iteration-joiner.dd
@@ -0,0 +1,19 @@
+The performance of `std.algorithm.iteration.joiner` has been improved
+
+$(H4 DMD)
+
+$(CONSOLE
+> dmd -O -inline -release ./joiner.d && ./joiner
+before.joiner   = 57 secs, 834 ms, 289 μs, and 3 hnsecs
+new.joiner      = 44 secs, 936 ms, 706 μs, and 5 hnsecs
+)
+
+$(H4 LDC)
+
+$(CONSOLE
+> ldmd -O3 -release -inline joiner.d && ./joiner
+before.joiner   = 5 secs, 180 ms, 193 μs, and 7 hnsecs
+new.joiner      = 3 secs, 168 ms, 560 μs, and 6 hnsecs
+)
+
+The benchmark code can be found $(LINK2 https://gist.github.com/wilzbach/ffd5d20639766a831fd4b1962572897a, here).
diff --git a/std/uni.d b/std/uni.d
index 0f9962c58..e52920236 100644
--- a/std/uni.d
+++ b/std/uni.d
@@ -9003,16 +9003,18 @@ if (isSomeString!S || (isRandomAccessRange!S && hasLength!S && hasSlicing!S && i
 {
     import std.array : appender, array;
     import std.ascii : isASCII;
-    import std.utf : byDchar;
+    import std.utf : byDchar, codeLength;
+
+    alias C = ElementEncodingType!S;
 
     auto r = s.byDchar;
-    for (size_t i; !r.empty; ++i, r.popFront())
+    for (size_t i; !r.empty; i += r.front.codeLength!C , r.popFront())
     {
         auto cOuter = r.front;
         ushort idx = indexFn(cOuter);
         if (idx == ushort.max)
             continue;
-        auto result = appender!(ElementEncodingType!S[])();
+        auto result = appender!(C[])();
         result.reserve(s.length);
         result.put(s[0 .. i]);
         foreach (dchar c; s[i .. $].byDchar)
@@ -9062,6 +9064,11 @@ if (isSomeString!S || (isRandomAccessRange!S && hasLength!S && hasSlicing!S && i
     assert(s == "abcdefghij");
 }
 
+@safe unittest // 18993
+{
+    static assert(`몬스터/A`.toLower.length == `몬스터/a`.toLower.length);
+}
+
 
 // generic toUpper/toLower on whole range, returns range
 private auto toCaser(alias indexFn, uint maxIdx, alias tableFn, alias asciiConvert, Range)(Range str)