Pardon my ignorance, questions about google number forwarding?????

If I port a very important old phone number number to google, to preserve it,
and then forward the calls to another cell number,
Will I encounter latency issues, as I do when using google voice and hangouts?
It is extremely important as we must preserve our original number, but face challenges using cell phone services with hard of hearing issues.

Quick answer YES but some get worse then others. Some can not tell difference
Some can

I've been using GV for several years and have forwarded calls to cell phones with Verizon, AT&T, T Mobile, and Sprint cell service and have not experienced any perceptible latency problems, yet , as you know, even without GV, people have problems with cell reception based on location and other factors.

I have recently moved my family's landline number they have had for over 20+ years to Google Voice. They are very thankful for the switch. My personal Google Voice Number I have had since 2012. I have not regretted moving to the service. All of my GV calls get forwarded to TruPhone (ATT MVNO). Before that was RingPlus and both have had extremely minimal latency issues.

I measured latency on Google Voice forwarding to another cell number: about 250ms attributed to Google Voice, plus whatever inherent latency of the cell carrier you're using. (Mine Sprint MVNO line at the time had about 80ms of "base" latency.)

It's not a problem for me, although I am aware of the delay.. I haven't heard complaints from other callers.
Although I haven't ever used it, I suspect that the latency from using GV in tandem with hangouts would be worse, since the 'base' latency of VoIP is most likely higher than that of cellular voice. I'm sure that there are users here who use GV with both cellular forwarding and hangouts, and could comment on the comparative latency.

My experience echoes Kent's. I've been using Google Voice since it first came out and it's the number that most people know, for me. (It spells out my name, which makes it very easy to remember.)