I didn't see the correct answer yet, so I'll just tell you, tabs have a delay to create because in almost all modern browsers you have to wait for a process to be created and for everything to be paged in, _then_ wait for a bunch of IPC calls to complete.
Old browsers didn't do this, so creating tabs was so fast! But they were also much easier to compromise. Tradeoffs.
Commenting just to add visibility to the only plausible answer I've seen to the question as posed. Thanks for not getting sidetracked into arguments about adblock/JS engines/layout/fonts/etc.
Why can’t they (a) have one process pre-created and ready for me before are press ⌘T or (b) don’t use a separate process to display an empty tab with Top Sites and create a process only after opening it? Or both :-)
Creating a process is not instantaneous but it is very quick - a small fraction of a second. And nothing should need paging in unless your system is under enough memory pressure to have paged things out.
If paging in is the problem then you need more memory (or fewer processes consuming your memory) or a fast SSD so that paging isn't slow. If you don't have a fast SSD then you should try to always have a few GB of RAM available, otherwise your system can't cache frequently needed resources and everything will be slow.
Creating a new tab in Chrome is very fast for me, even on my six-year-old laptop. I can type Ctrl+T and instantly be typing a new URL/search-term.
There are hundreds of things that can make tab creation slow - anti-virus, low-memory, malware, extensions, CPU contention, etc. It is likely that different people on this thread are having their browser experience slowed by different things. Only by monitoring your specific system, or performing experiments (try disabling all extensions) can you find out why browsing is slow for you.
In some cases the slowdowns could be bugs in the browsers which happen to only affect some people, but in most cases I think it is external factors.
chrome://tracing can be used to investigate slowdowns in Chrome. ETW tracing (see UIforETW) can be used to investigate slowdowns on any Windows machine.
Old browsers didn't do this, so creating tabs was so fast! But they were also much easier to compromise. Tradeoffs.
Source: Electron maintainer