How many events do you have in there?

Do you see any errors in the TB console?

It happens when you add +1000 events.

Nothing can be seen in the console, thunderbird just stops responding.

It happens for some of our users at our company here, too, and we believe that it's a Lightning bug - it seems it just blocks Thunderbird when importing events until it is done.

Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/chrome.manifest'.

Timestamp: 12/09/2014 12:23:53
Warning: WARN addons.xpi: Ignoring missing add-on in C:\Program Files\McAfee\MSK
Source File: resource://gre/modules/XPIProvider.jsm
Line: 6986

Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/extensions/%7B972ce4c6-7e08-4474-a285-3208198ce6fd%7D/chrome.manifest'.

While creating services from category 'profile-after-change', could not create service for entry 'Disk Space Watcher Service', contract ID '@mozilla.org/toolkit/disk-space-watcher;1'

Timestamp: 12/09/2014 12:23:58
Warning: Use of getAttributeNodeNS() is deprecated. Use getAttributeNS() instead.
Source File: resource://calendar/modules/calXMLUtils.jsm
Line: 32

[JavaScript Warning: "Use of doQueryInterface() is deprecated and will be removed with the next release. Use XPCOMUtils.generateQI() instead.
1: [resource://calendar/modules/calUtils.jsm -> file:///C:/Users/irinat/AppData/Roaming/Thunderbird/Profiles/eg26sry9.default/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/calendar-js/calUtils.js:826] doQueryInterface
2: [jar:file:///C:/Users/irinat/AppData/Roaming/Thunderbird/Profiles/eg26sry9.default/extensions/sogo-connector@inverse.ca.xpi!/components/CalDAVACLManager.js:1411] cDACLM_QueryInterface
3: [resource://gre/modules/XPCOMUtils.jsm:271] null
4: [resource://calendar/modules/calProviderUtils.jsm:594] null
5: [file:///C:/Users/irinat/AppData/Roaming/Thunderbird/Profiles/eg26sry9.default/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/components/calDavCalendar.js:1304] caldav_safeRefresh
6: [file:///C:/Users/irinat/AppData/Roaming/Thunderbird/Profiles/eg26sry9.default/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/components/calDavCalendar.js:2217] caldav_completeCheckServerInfo
7: [file:///C:/Users/irinat/AppData/Roaming/Thunderbird/Profiles/eg26sry9.default/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/components/calDavCalendar.js:2181] caldav_cPNS_oSC
8: [null:0] null
"]

Lightning issue. We can't parse events for Lightning.

SOGo can return all calendar data (1000 events) in 0.2 second of a small server.