Joomla is no slower than other CMSystems. There will always be a trade off in speed when convenience of solution development is sought after.
I'd like to assure you that Joomla's performance does not automatically take a hit as the size of the system increases.
There are any number of very large portal / websites / applications that are delivered that have excellent content delivery times even under modest concurrent hits.
All to often regretfully, developers who use Joomla, completely ignore the basics associated with page load speeds when crafting an Internet delivered solution.
That bound to shared hosting, on low end servers, can combine to make a Joomla mounted enterprise wide application appear to be slow.
Anyways the information you've provided is too little to give any specific reasons to you.
If you could provide a URL to check out that would help.