The release came at the same time that the company announced it was close to double its bookings for 2020 compared to 2019.
A statement from the company said the improvements in version 21 included optimisations for app loading and database queries.
There are new client preview features that will improve the response times for mobile clients and the search has been optimised for a number of apps.
"Nextcloud 21 will reduce the load on servers, enabling more employees to work without requiring additional hardware. At the same time, the new collaboration features improve the day-to-day productivity and support team members in getting more work done in less time.
The new back-end is an optional component that can retain a direct connection with desktop and web clients, feeding updates on file changes and notifications to clients.
A collaborative whiteboard app allows users to draw, take notes and upload images during a call with a client.
Nextcloud Hub 21 is compatible with the new PHP 8 and will not work with PHP 7.2.
Roeland Douma, Nextcloud's Server Engineering and Support lead, said: "At a scale of millions of users, milliseconds start to add up. After analysing application and caching server, storage and database behaviour, our team was able to significantly reduce the impact of common operations.
"The High Performance Back-end, on the other hand, provides a completely new way of reducing server load while bringing a new level of responsiveness to users.
Regarding the growth of users, the company said the release of version 20 of Nextcloud Hub had led to a tripling of downloads and doubled lead flows.
"With a 50% growth in customer base and a near doubling of total bookings, Nextcloud managed a healthy growth in average subscription value," the company said.
"A continued strong focus on subscriptions over consulting and an increase of the minimum base subscription to 100 users is expected to further improve these numbers over 2021.
"The sales team has expanded by 50% to service new customers and this growth rate is planned to continue."