Is there a way to update the height of a single UITableViewCell, without recalculating the height for every cell?

As noted, reloadRowsAtIndexPaths:withRowAnimation: will only cause the table view to ask its UITableViewDataSource for a new cell view but won’t ask the UITableViewDelegate for an updated cell height.

Unfortunately the height will only be refreshed by calling:

[tableView beginUpdates];
[tableView endUpdates];

Even without any change between the two calls.

If your algorithm to calculate heights is too time consuming maybe you should cache those values.
Something like:

- (CGFloat)tableView:(UITableView *)tableView
heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    CGFloat height = [self cachedHeightForIndexPath:indexPath];

    // Not cached ?
    if (height < 0)
    {
        height = [self heightForIndexPath:indexPath];
        [self setCachedHeight:height
                 forIndexPath:indexPath];
    }

    return height;
}

And making sure to reset those heights to -1 when the contents change or at init time.

Edit:

Also if you want to delay height calculation as much as possible (until they are scrolled to) you should try implementing this (iOS 7+ only):

@property (nonatomic) CGFloat estimatedRowHeight

Providing a nonnegative estimate of the height of rows can improve the
performance of loading the table view. If the table contains variable
height rows, it might be expensive to calculate all their heights when
the table loads. Using estimation allows you to defer some of the cost
of geometry calculation from load time to scrolling time.

The default value is 0, which means there is no estimate.