T57W LDAP delays

[wp92]

hello!

We have the following issue in our setup (10x T57W with a cloud provider):
When opening the LDAP contacts on the phone there is almost always a delay, sometimes 20 seconds and more before anything is shown at all.
We have done all kinds of checks on the network, the problem does not seem to be there.
Also with the softphone client of the provider these problems dont show.
The LDAP Server is from the provider so accessed over the internet.
Thanks in advance for any help solving or at least further diagnosing this.
PS: Why is there no cache for global contacts on the phone, wouldnt that be way better also to keep constant queries away more from the LDAP server?