perf: slice is faster

[cesco69]

https://www.measurethat.net/Benchmarks/Show/2335/1/slice-vs-substr-vs-substring-with-no-end-index#latest_results_block

slice x 25,768,061 ops/sec ±10.38% (42 runs sampled)
substr x 23,764,229 ops/sec ±8.05% (42 runs sampled)
substring x 24,512,322 ops/sec ±10.47% (29 runs sampled)