If only LR is affect, then it doesn’t sounds like the M&S security fix causing the performance degradation but something else (may be GPU driver, may be other updates, but inside the same system update package).
If that’s due to the security patch greatly reduce the speculative execution performance, that effect should be much more noticeable, but not only happening in LR.
I didn’t use LR for a while already, so can’t tell the difference now, but may try it a bit later to see if my Mac also painfully slow when using it (in 10.13.3). LR 6 was quite smooth on my machine since it was launched.
However, at the moment, I can’t see any performace degradation in general on my Mac. All video editing, ecoding, gaming, benchmarking are more or less the same.
I am with the old Xeon, so, should be affected more than any newer i7. In 10.13.2, that quickly released security patch seems slow down my ~9 years old Mac for about 5-10% in benchmarks, but still can’t quite feel the difference in daily usage. The 10.13.3 update actually bring the performance back to normal level (IMO, 1% difference is within the normal measuring error in any wildly used benchmarking software.
So, i personally tends to say that the relative security patch didn’t actually cause any noticeable performance degradation.