Write Back is not necessarily faster than write through. It adds additional latency time when data from the CPU should be written to main memory. If the code is arranged the way that it takes write back into account preventing unnecessary flushes of cache lines to main memory then it can be much faster.
On my tests with a 486DX2-66 with WT and WB it showed that most DOS games run a few percent faster with WT. However on a DX4-100 the clock is higher and thus latency time lower. This favors WB mode.