Hi,
Shared Hosting : An Account to manage single or multiple domains
Reseller Hosting : An Account to create, manage multiple shared hosting accounts.
If I upgrade to reseller hosting, that will improve performance and increasing limitations on my shared account?
In case of upgrading to reseller hosting at same provider, That depends on how your hosting provider built Shared hosting and reseller hosting packages.
case 1 : You are upgrading to reseller hosting and creating one shared hosting account to manage your respective domain.
If your provider has same features and limitations for directly created shared hosting accounts and shared hosting accounts created via reseller hosting; You are going to have same limits.
If you are facing resource limitation issue, please clarify.
In most cases, moving to own virtual machine is an option.
Why don't you clarify more details, like what exact scenario of your problem. That will help us provide accurate answer.