depending on your audio interface, this is where latency exists and secondly, CPU power for processing the amp model.
Here's a trick I read before, split your signal, and have a dry signal going to PC and other signal to your amp to get sound while recording.
One suggestion is ART Tube MP ($50)
Then, you apply Amp Model "after" ... tweek away until you find the sound you want to keep.
Another trick, even after you find the sound, you can Render the track as an audio track, and just Mute the original = 0% CPU
You can then still go back and mess with your amp model and FX.
But here's where it get's interesting. Since you created an audio track, make the original track different, change it up and Render to another audio track. Now, mix audio track 1 and 2 into your mix, and you may get a fuller guitar tone. Experiment.
Suggestion. There are a few Free ones out there. AmpliTube Free, Guitar Rig Free, and this freeware SHRED
Shred 1.0 :: Free VST Amp Suite | AcmeBarGig
BTW, I use all three and my Zoom G3 processor ($200)