Yes, that is exactly the issue as the interchange time is insufficient and there are potential safety issues with parts of the route especially in hours of darkness. Fratton (lifts) and Portsmouth & Southsea (cross platform interchange in most cases) are much better options.
The National Rail website states that:
Step-free category B2 Station. Step-free access is possible to both platforms via short steep ramps (up to 1:5 gradient in places, but generally 1:9). Interchange between platforms is either via footbridge with steps, or via long step-free route using cycle lane, crossing Norway Road and secluded path.
The cycle lane is up a steep gradient heading away from the station:
Find local businesses, view maps and get driving directions in Google Maps.
www.google.com
Then there is a busy road bridge with a narrow footway back over the railway:
Find local businesses, view maps and get driving directions in Google Maps.
www.google.com
...and finally cross the busy road with no proper crossing to go down the secluded path behind the yellow bin which takes you back under the main road and runs parallel with the railway to the up platform.
Find local businesses, view maps and get driving directions in Google Maps.
www.google.com