Update css class on iOS slow

Ionic Framework ionic-v3
1

Hi all,
I face a strange problem. I have an ion-row where I display different numbers horizontaly:

            <ion-row no-padding align-items-center>
                <ion-col class="times-col"  *ngFor="let item of selectedProgramm.availableTimes; let i = index" (click)="selectTime(i)">
                  <div class="time-choose" [class.time-choose-selected]="selectedTimeIndex === i"><p>{{item}}</p></div>
                </ion-col>
            </ion-row>

On my old Android 6 device the selection runs fast. But on my iPhone6s there is a lag of around a sec for updating the selection.

in my ts I only update the selected index:

  selectTime(item){
    this.selectedTimeIndex = item; 
  }

My css:

    .time-choose {
        width: 80px;
        height: 50px;
        border-radius: 100px;
        font-weight: 300;
        color: color($colors,dark);
        display: block;
        margin: auto;
        margin-top:-20px;
        p {
            line-height:50px;
            font-size: $font24;
            text-align: center;
        }
    } 

    .time-choose-selected {
        color: color($colors,clear);
        background-color: color($colors,primary);        
    }

Any hints where the lag can come from?

thx,
patrik