What's specifically happening when a browser starts slowing under tab load?

[warmachine2]

Browsers start to get sluggish and unresponsive under increasing tab load. What/Where specifically is the source of this slowness, and could it possibly be "boosted" in some way (other than limiting tabs)?

Obviously, there's a heavy load on memory when you have 100+ tabs open. But if there's a copious amount of RAM + page file available, why does it still get slow? What function or mechanism is the source of the slowdown? Is it on the browser's side of responsibility, or the OS'?

Without the Firefox source code, I'm wondering if there's some measure I could tweak to circumvent the slowdown, like perhaps manually allocating metric-ton of disk cache, or something.

(For reference, speaking of 64-bit browsers)

[Jeremy Collake]

That is a big question. Put simply, modern web pages are 'heavy'. They have lots of elements and scripts. The browser (and OS) try to keep memory that might be soon referenced in RAM. You want your RAM to be as utilized as possible as the fastest storage medium on your device. Further, just because a tab isn't visible doesn't mean it is not doing anything.

This is an area of optimization targeted by entire teams of developers, so you aren't going to find a simple tweak to make. Best case scenario, you change the browser settings to behave more like what you think you want, but there will be some trade-off to that which may not be immediately apparent to you.

[kanetracy]

If your computer is running multiple applications at once, consuming a lot of memory and processing power, the browser may experience slowdowns when attempting to load new tabs.
The browser may experience difficulties loading, processing, and displaying a tab with excessively complex content, such as a web page with numerous photos or videos or an application that requires a lot of processing power. This phenomenon is known as tab overload.
Extensions for your browser: When loading data or handling duties associated with the current tab, some extensions could slow down your browser. Delays may occur on sometimes because this add-on isn't always compatible with the browser or has issues. 
Browser or OS updates: Sometimes, when you update your browser or OS, there can be compatibility issues that cause tabs to load slowly.

[cynthiaking]

Memory Usage: Each open tab consumes a certain amount of RAM. As the number of tabs increases, the browser requires more memory. If the available RAM is insufficient, the system starts using virtual memory, which is slower and leads to a performance drop.

CPU Load: Each tab may have active processes, such as rendering web pages, running JavaScript, and handling multimedia content. A high number of active tabs can overwhelm the CPU, causing it to struggle with the workload.